Daylight distribution in a daylight and sunlight report

Daylight distribution, assessed through the no sky line, answers a question the window test cannot: how far into a room does daylight actually reach. Vertical sky component measures the amount of sky visible at the outside face of the window. Distribution measures the proportion of the working plane inside the room from which any sky can be seen. A window can hold its vertical sky component and still lose a third of its room to a new building opposite, because the obstruction has raised the angle at which sky first appears.

That is why BRE 209 treats them as two tests rather than one, and why reporting only the window result invites a challenge.

What distribution measures that the window test does not

The no sky line is the boundary on the working plane between the part of the room that can see some sky through the window and the part that cannot. Everything beyond that line receives no direct skylight at all and depends on interreflected light.

BRE 209 gives the guideline in relative terms. Where the area of the working plane receiving direct skylight is reduced to less than 0.8 times its former value as a result of a development, the loss will be noticeable to occupiers. That ratio is the number officers look for, alongside the vertical sky component guideline of 27 per cent or 0.8 times the former value where the existing figure is already below it.

The working plane for residential assessment is taken at 0.85 m above floor level, which is desk and table height, and the line is plotted in plan for each assessed room.

For rooms within a proposed building the current edition of BRE 209 points towards the illuminance-based approach in the European daylight standard and its UK national annex, with target illuminance levels varying by room type and required across a proportion of daylight hours. Keep the two frameworks apart in the report. Existing neighbouring rooms are assessed against the relative guidelines. Proposed rooms are assessed against the target illuminance criteria.

Plotting the no sky line

The calculation is geometric. For each point on the working plane, determine whether a ray from that point through the window clears the obstruction and reaches the sky. Modern software does this automatically from the massing model, which makes the inputs the only thing worth checking.

Four inputs govern the output:

• The window geometry, including reveal depth, transom and mullion positions and any obstruction from a balcony above
• The room geometry, particularly depth from the window wall and ceiling height
• The external obstruction model, including the existing situation, the proposed development and any consented schemes
• The working plane height and the extent of the room treated as the working plane

Reveal depth and balconies are the two most often simplified away. A deep reveal on a converted warehouse and a balcony slab over a window each cut the visible sky, and a model that treats the facade as flat overstates the baseline, which flatters the result.

Room depth and its effect

Distribution is unforgiving of deep rooms. In a room 4 m deep with a reasonable window, the no sky line sits well back and a modest obstruction moves it a little. In a room 9 m deep lit from one end, the line sits closer to the window as a proportion of the floor area, and the same obstruction can take a large share of the remaining lit area.

Open plan living, kitchen and dining rooms created by removing an internal wall are the hardest case. The combined room is deeper than either of its parts, its no sky line covers a smaller proportion of the floor area at the outset, and the relative test is applied to that weaker baseline. It is a known limitation of applying a room-based test to a room the guidance did not anticipate.

Where a room has windows on more than one wall, distribution usually holds up far better than the window measure suggests, because the second aperture lights the area the first cannot reach. Model it rather than assuming it.

Furniture, partitions and assumed layouts

The assessment depends on the room layout, and on existing neighbouring properties the layout is often unknown.

The convention is to assume the layout as existing, established from a site visit, from historic drawings, or where neither is available from a reasonable assumption based on the building type. State the assumption for every assessed room. Where access was refused, say so and say what was assumed instead, because an assumed layout that later proves wrong is the most common route to a challenged conclusion.

Furniture is not modelled. Partitions are, because they define the room. Where a property has been altered without permission, for example a conservatory added in front of a window, the assessment normally reports both the lawful and the actual situation, which is more useful to a decision maker than picking one.

The same discipline applies to any modelling exercise where the inputs are more contestable than the method, which is the recurring theme in modelling assumptions for an overheating risk assessment as well.

Reporting area proportions clearly

Report distribution as areas rather than as a pass or fail.

The plan matters as much as the number. A room retaining 0.79 of its lit area where the loss falls across a corner reads differently from one where the loss takes the whole area around the dining table, and an officer can only see that from a drawing.

Report every assessed room rather than a summary of those that fail. A table showing 40 rooms comfortably compliant and three marginal is a stronger document than one presenting only the three.

When distribution matters more than the window result

Four situations where distribution is the test that decides the case.

Deep single-aspect rooms, for the reasons above. Rooms lit through a window under a balcony or a deep overhang, where the sky visible is already limited to a narrow band and a new obstruction removes it entirely. Rooms where the new obstruction is asymmetric, such as a side extension beside one end of a window, which can leave the vertical sky component largely intact while cutting the lit area at one side of the room. And ground floor rooms facing a courtyard, where the angle to the sky is shallow to begin with.

In each of those, a report that leads on vertical sky component and mentions distribution in an appendix has buried the point that matters.

Combining the measures into one conclusion

A daylight and sunlight assessment usually reports three things for neighbouring properties: vertical sky component at each window, daylight distribution for each room, and annual probable sunlight hours for windows within 90 degrees of due south. Amenity areas are assessed separately, with the guideline that at least half the area should receive at least two hours of sunlight on 21 March, which is where an overshadowing study comes in.

The conclusion has to reconcile them rather than list them. Where the measures disagree, say which carries more weight in this case and why, with reference to the room and its use. And apply the guidance as guidance. BRE 209 states that its figures are advisory and should be interpreted flexibly according to the context, which in a dense urban area with an adopted plan seeking higher densities means the numeric guidelines are a starting point rather than a standard.

An assessment that reports a technical shortfall, explains its practical effect on the occupier and sets it against the character and policy context is doing the job. One that reports compliance against a target and stops is leaving the planning judgement to somebody else.

To see which supporting documents a scheme is likely to need, enter the postcode on PlanScope and work from the report list it produces.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Councils usually expect a suitably qualified author for daylight and sunlight work, and professional review is strongly advised.